This invention relates to a safety relief system for a valve in a vent or a main flowline, and more particularly to a safety relief system utilizing a vacuum boost actuator connected to the valve for opening of the valve under a negative pressure.
Heretofore, safety relief systems have utilized a control valve in the main flowline having a main diaphragm actuator with the dome or upper fluid chamber of the actuator in fluid communication with an associated pilot valve which senses upstream fluid pressures in the flowline thereby to control the movement of the main valve member across the flowline.
The main actuator heretofore while responsive to a negative pressure to open the main valve member does not respond to a minimal negative pressure in view of the relatively small effective fluid pressure differential area exposed to the negative fluid pressure. When desired heretofore to open under a minimal negative fluid pressure condition, a separate so-called vacuum breaker or vacuum device has been provided for the main flowline having a separate body and valve member for relief of the vacuum condition in the flowline. Such a separate vacuum device is expensive and has additional space requirements which is not suitable for certin installations. In the event a separate vacuum relief device is not provided, a much higher vacuum is required for the main actuator to open the valve member than is recommended by pertinent associated code requirements or specifications such as ASME codes or the like, for example.
A main diaphragm actuator may be designed for a set pressure of around 50 psi or higher, for example, and must have the capability of lifting the main valve member and associated attached moving parts. Likewise, upon the exertion of a negative pressure, the same main valve member and associated moving parts must be lifted and, therefore, a high vacuum is required for opening of the main valve member. Thus, when a relatively small differential pressure area is provided, a relatively high vacuum is required to open the main valve member.
Regulator valves or the like have utilized two or more diaphragm actuators heretofore. For example, U.S. Pat. No. 1,956,977 dated May 1, 1934 shows a regulating valve in which a pair of diaphragms are connected to a valve stem for controlling movement of a main valve member but the diaphragms have a common fluid chamber and both diaphragms move with the main valve member under positive pressure.
U.S. Pat. No. 863,528 dated Aug. 13, 1907 shows a fluid pressure regulator in which three diaphragms are utilized to permit a travel of the main valve member twice the travel of the diaphragms thereby to cause the regulator to respond quickly to fluid or gas changes in the main flowline.
U.S. Pat. No. 3,256,903 dated June 21, 1966 shows a vacuum regulator in which separate vacuum actuators are connected to a valve stem and are responsive to a negative pressure for movement of a main valve member. Both actuators are vacuum actuators, however, and are not designed for normal movement of a main valve member under positive pressure.